Added GrantTrait::setIdentifier

I found it useful to be able to set the identifier so I could "alias" one for deprecation. Hopefully no issues here @alexbilbie

src/League/OAuth2/Server/Grant/GrantTrait.php

@@ -22,6 +22,17 @@ public function getIdentifier()
         return $this->identifier;
     }
 
+    /**
+     * Return the identifier
+     * @param string $identifier
+     * @return self
+     */
+    public function setIdentifier($identifier)
+    {
+        $this->identifier = $identifier;
+        return $this;
+    }
+
     /**
      * Return the response type
      * @return string
@@ -42,4 +53,4 @@ public function setAccessTokenTTL($accessTokenTTL)
         return $this;
     }
 
-}
+}